The present invention relates to nozzle guide vane assemblies, particularly the inlet nozzle of a large gas turbine engine, and methods for assembly of such nozzle guide vane assemblies into a gas turbine engine.
A nozzle guide vane assembly is part of the stationary (non-rotating) structure in an axial flow gas turbine engine. In overall configuration, this assembly comprises concentric outer and inner ring-like nozzle bands defining an annular space therebetween, which annular space is a portion of the hot gas path through the gas turbine engine. A plurality of air foil-shaped vanes extend generally radially between the nozzle bands, nozzles being defined between the vanes.
Stationary nozzle guide vane assemblies are either inlet nozzles or interstage nozzles. An inlet nozzle is conventionally positioned on the turbine axis between the gas turbine combustor and the first stage turbine rotor, and serves to direct motive fluid in the form of high temperature combustion products against the rotor blades (also termed "buckets") at a proper angle. Interstage nozzle guide vane assemblies perform a similar function, but are located along the turbine axis between individual rotor assemblies. The present invention is primarily concerned with gas turbine inlet nozzle assemblies because the various thermal and mechanical stresses are most severe in that location. However, the principles of the invention are also applicable to interstage nozzle guide vane assemblies.
The nozzle bands are called by various terms in this art, such as nozzle shrouds. However, for clarity and consistency, herein these bands are termed inner and outer nozzle "endwall rings" since they are in effect located at the ends of the nozzle vanes which extend between the endwall rings.
Except for very small gas turbines, it is impractical to make the entire nozzle guide vane assembly a single rigid lattice with continuous inner and outer endwall rings and vanes fixed at both ends to both endwall rings. This is because stresses occasioned by thermal expansions and accompanied by gas loading result in dispersion and ultimate destruction of the nozzle structures.
One conventional approach to this problem is to form the nozzle guide vane assembly as a plurality of individual arcuate nozzle segments individually retained by a separate ring-like support shroud or casing. Seal structures are typically provided between adjacent segments where necessary. The arcuate nozzle segments may be either one-vane or multi-vane, and each nozzle segment generally comprises one or more vanes extending between a nozzle segment inner endwall and a nozzle segment outer endwall. In the completed structure, generally circumferentially-facing edges of each nozzle segment are located adjacent to corresponding circumferentially-facing endwall edges of adjacent nozzle segments to form continuous inner and outer endwall rings.
Various other approaches to this problem have also been proposed. For example, in one particular other approach, the nozzle vanes are formed separately from one or both of the endwall rings, and the endwall rings may themselves either be one-piece or formed from individual endwall ring segments. Specific design proposals embodying this general type of structure include various specific forms of attachment of the nozzle vanes to the endwall rings, typically permitting relative movement, either relative radial movement or pivoting movement.
As mentioned above, in the type of nozzle guide vane assembly wherein a plurality of arcuate nozzle segments are adjacently located, the segments are individually retained by a separate ring-like support shroud casing structure. Although a full nozzle ring is defined, gaps between the generally circumferentially-facing edges of adjacent segments are necessary to allow for manufacturing tolerances, and to accomodate thermal expansion. As turbine operating temperature is increased, the hot gas leakage through these gaps produces intolerable heating of the edges, particularly for a water-cooled composite nozzle designed to expose to hot gas only the surfaces forming the aerodynamic path. Leakage also has an adverse effect on efficiency. In view of these considerations, some relatively complex sealing arrangements have been proposed. Typically, however, strips of metal provide sealing.
However, in a nozzle guide vane assembly comprising high temperature, water cooled segments, water-cooling the seals is not practicable. Air cooling would not only result in inefficient operation, but would impose severe thermal gradients in the nozzle segments.